Variation in level of growth of RS virus was detected for 20 strains of inbred mice tested. The difference in level of replication between the most permissive and restrictive strain of mice was sufficiently large that it should now be possible to analyze genetic control of viral replication by the appropriate cross-breeding techniques. ts-2, ts-1 NG-1 and ts-1 NG-16 mutants were shown to be highly defective and attenuated when studied in primates suggesting that these mutants may prove useful in prevention of human RS virus disease. IM inoculation of virus appears to initiate an aborative cycle of replication locally leading to an immunologic response. Circulating antibody acts to suppress this response. Development of a glucose oxidase linked antibody staining technique offers the possibility of examining paraffin embedded, formalin fixed tissues for presence of RS antigens. In this manner an immunopathoarcheologic survey of RS virus and other viral pathogens can be performed using autopsy material collected previously.